Overhaul raindrop water ripple effect

• Replace raindrop ripple math with math that more clearly generates actual ripples.
• Make raindrop ripple random placement change over time instead of only over different positions
• Make the forced specularity/whiteness/lightness of water ripples use the absolute height of the ripple instead of the height itself. Doing max(0, ...) would technically be better but the difference isn't significant and abs() is shorter. Willing to change this to max(0, ...) if desired.

This looks slightly weird with refraction water (some ripples look "brown"), but only because refraction water itself handles estimated sunlight scattering improperly (the "brown"ness is only because those parts the ripples aren't green like their surroundings).

Willing to re-rewrite the math if the new math is too long/verbose to be wanted.

Closes #6360 (closed).